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Use a small Dremel fiber/brass wire brush to clean the grooves.

Typo, haha, picks! 
www.harborfreight.com/test-probe-set-5-pc-61872.html

Or Sciroccos method. Get a dremel with the flex shaft. I’m amazed at the fact it usually my go to for so many tasks.


 

Alright, will do. I was worried about scuffing up the inside and making my problem worse. 

I’d suggest using the wire wheels. The brass won’t scratch. Then gently pick out the stubborn crud with the picks. Then a brake clean spray down. And more inspections till the grove is clean. It won’t hurt to use the wire wheels on the main piston bore to clean up heavy crud. Make sure you have no pitting on the pistons. 

Pistons are brand new. The bore is actually pretty clean. The groove is the dirty part. 

Mikaw post=849948I’d suggest using the wire wheels. The brass won’t scratch. Then gently pick out the stubborn crud with the picks. 

I've been using Dremel stainless steel brushes.  They work well, and don't seem to be too aggressive for the aluminum.

amazon.com/Dremel-530-02-Stainless-Steel-Brushes/dp/B015PK5DRY

For a pick I've been using a softer pick that doesn't seem to scratch as much as the hard pointy ones.  A magnet does stick to it, but it bends pretty easy.  I think it's a pottery tool.

I clean the bore and seal groove with brake cleaner first.  It makes the crusty stuff stand out white against the aluminum.  Then I go after the bigger bits with the pick, and then clean out the groove with the stainless wheel.  I also have been using 1200 grit wet sand paper (wet with water) to clean stains off the inside surface of the piston bore.  

DOHC wrote:

Mikaw post=849948I’d suggest using the wire wheels. The brass won’t scratch. Then gently pick out the stubborn crud with the picks. 

I've been using Dremel stainless steel brushes.  They work well, and don't seem to be too aggressive for the aluminum.

amazon.com/Dremel-530-02-Stainless-Steel-Brushes/dp/B015PK5DRY

For a pick I've been using a softer pick that doesn't seem to scratch as much as the hard pointy ones.  A magnet does stick to it, but it bends pretty easy.  I think it's a pottery tool.

I clean the bore and seal groove with brake cleaner first.  It makes the crusty stuff stand out white against the aluminum.  Then I go after the bigger bits with the pick, and then clean out the groove with the stainless wheel.  I also have been using 1200 grit wet sand paper (wet with water) to clean stains off the inside surface of the piston bore.  

All good advise. I like it. I have both stainless and brass wire wheels. And red, green, black, and gold scotch bright wheels for the dremel. 

I have cleaned out the seal groove with a brass Dremel brush, reinstalled new seals and pistons with red grease, installed my new dust boot rings,  reassembled the remaining brake pieces, added brake fluid, and bleed out the system. I am now getting good pressure at the brake handle and everything seems to be working well. No leaks that I can see, so I really appreciate the help! I will say that this assembly was totally different than the first time after using the red grease. The first time I had to use a vice to push the piston into the cylinder. This time it went in with minor hand pressure. Hopefully this $200 investment will get me another 40+ years of good safe service!

 

Yay!

One more comment. You can track the fluid lever in the M/C reservoir. Or you can pull the caliper and lift the dust seal and look for signs of brake fluid. If have consistent lever travel after sitting a few days I’d call it good. 

So, far so good. I still see no leaking and same travel on the lever. The bike does not run, so I can't put it through its paces just yet, but I've been giving it a good squeeze and pumping it the way I would if I were on the road. Not much else I can do.